These are 3 Isochrones created with the OSM-Tool. They are marking catchment areas for footwalking. Since im pretty new to QGIS I have no idea how to unite the isochrones into 1 Isochrone. Any idea?
on the top menu use vector > geoprocessing tools > dissolve. On the tool's dialogue box, uncheck the "dissolve all" check box and add the attribute that defines your isochrones in the "Unique ID fields" "selected" box. You can save the output to a shapefile or keep it as a temporary layer in the last box.
Take a look here for a video tutorial on the dissolve tool